Hepatic stellate cells (HSCs) play a pivotal role in hepatic fibrogenesis and are considered a major cellular target for therapeutic intervention. However, no approved anti-fibrotic drug is currently on the market.
Hepatic fibrogenesis may be incurred as a result of insults arising from oxidative stress, chemical toxicity or viral infection. Thus fibrotic disease, including cell and tissue fibrosis, is among the physiological disorders that have been associated with oxidative stress. For example, oxidative stress resulting from the metabolic generation of ROS has been linked to HSC activation and liver fibrosis (Britton et al. 1994, Tsukamoto et al. 1995, GAB et al. 2005). It has been shown that products of lipid peroxidation lead to increased collagen synthesis by HSCs (Casini et al. 1997 and Parola et al. 1996).
Reactive oxygen species (ROS) are oxygen derivatives that include free radicals and non-radical reactive molecules such as peroxides and oxygen derivatives. Oxidative stress occurs when there is an excess amount of ROS due to an imbalance between the generation and the elimination or neutralization of these molecules. Excess amounts of ROS may damage cell lipids, proteins and DNA resulting in the inhibition of normal cell function or the development of abnormal cellular behaviour which can in turn lead to a variety of diseases and conditions, depending on the cells affected. ROS have been implicated in many physiological disorders and in the onset and development of a number of diseases.
Some anti-oxidants have been explored as potential inhibitors of hepatic fibrosis (Kawada et al. 1998). (−)-Epigallocatechin gallate (EGCG), a major active component of tea catechin, has been shown to inhibit HSC activation in vitro via transforming growth factor-beta (TGF-β) signalling (Chen et al. 2002, Nakanuta et al. 2005, Fu et al. 2006). In clinical studies, a combination of vitamins E and C was shown to decrease the fibrosis score in non-alcoholic steatohepatitis patients, but did not affect hepatic inflammation (Harrison et al. 2003). N-acetyl-L-cysteine (NAC), a synthetic precursor of glutathione (GSH) that has been clinically used as an antioxidant, showed anti-fibrogenic properties through the suppression of TGF-β signalling transduction as well (Meurer et al. 2005).
Oxidative stress has also been associated with the onset and progression of cancer. It has been reported that oxidative stress as indicated by reduced anti-oxidant enzyme activities is associated with the development of primary carcinogenesis and metastasis in clinical patients (Vali et al. 2008). ROS have been found to cause DNA damage increasing the risk of DNA mutation and thus the development of cancer (Hussain et al. 2005). Tea polyphenols are anti-oxidants and have been shown to inhibit carcinogen-induced DNA damage in animal models of skin, lung, colon, liver and pancreatic cancers (Frei et al. 2003). In addition, anti-oxidants PBN and NXY-059 have demonstrated anti-cancer activity in hepatocellular carcinoma. (Floyd 2006). However, the effectiveness of the anti-oxidants investigated remains unclear (Valko et al, 2004).
The inflammatory response is the immune system's response to infection or injury and involves the activation of cells of the immune system which produce mediators, such as cytokines, that further activate other cells, leading to a cascade of immune reactions that fight off the infection or repair the injury. Like oxidative stress, inflammatory stresses have been associated with fibrotic disease and cancer (Rakoff-Nahoum 2006; Tsukamoto et al. 1999; Bachem et al. 1992; Vasiliou et al. 2000). For example, in the development of liver fibrosis, HSCs play a critical role. Responding to liver injury, HSCs undergo a process called “activation” and trans-differentiate to myofibroblast-like cells. This process is characterized by phenotypic changes including cell proliferation, over-expression of smooth muscle actin-α (SMAA), and deposition of extracellular matrix (ECM) proteins, including collagen type αI (I) (col1a1) and fibronectin.
Inflammatory cytokines represent major mediators for HSC activation. Among them, transforming growth factor-beta I (TGF-β1) and interleukin 6 (IL-6) have been categorized as profibrogenic cytokines mainly responsible for the induction of ECM proteins (Tsukamoto 1999). HSCs respond to TGF-β1 secreted from Kupffer cells and endothelial cells during liver injury, and themselves via autocrine action, resulting in HSC activation and liver fibrosis (Bachem et al. 1992). In addition, the activation of HSCs can also be attributed to chronic hepatic inflammation through the secretion of proinflammatory cytokine IL-6, leading to cirrhosis (Vasiliou et al. 2000) and hepatocellular carcinoma (Naugler et al, 2007). Transcription factor nuclear factor kappa B (NF-κB) is an important regulator for the secretion of inflammatory cytokines, and its subunit NF-κB p65 was reported to mediate liver fibrosis (Vasiliou et al. 2000).
Beyond sharing a common association with fibrotic diseases and cancer, the production and regulation of oxidative and inflammatory stress appear to be interconnected, as an inflammatory response can trigger oxidative stress and vice versa.
For example, in hepatic fibrogenesis, both ROS and pro-inflammatory cytokines have been shown to be involved in the activation of HSCs, the central event in the development of liver fibrogenesis. TGF-β is a major mediator for HSC activation and TGF-β signalling is affected by both oxidative stress and the induction of an inflammatory response (Tsukamoto et al. 1999, Bachem et al. 1992, Chen et al. 2002, Nakanuta et al. 2005, Fu et al. 2006, Meurer et al. 2005). Another key molecule in the development of fibrosis, is NF-κB which is an important regulator of the secretion of inflammatory cytokines but is also known to be sensitive to oxidative stress. Most agents activating NF-κB are either modulated by ROS or oxidant themselves. It has been reported that treatment with anti-oxidant resveratrol (Chavez et al. 2007) or vitamin E (Liu et al. 1995) attenuated NF-κB elevation induced in carbon tetrachloride experimental fibrotic rodents.
Oxidative stress and inflammatory response have also been found to be interrelated in the pathogenesis of cancer. Oxidative stress can cause DNA mutations, some of which will result in the formation of cancer cells. However other mutations will result in cell death, stimulating an inflammatory response. In turn, an inflammatory response will not only provide survival and proliferative signals to cancer cells but may also induce the production of ROS (Rakoff-Nahoum 2006).
Dietary anti-oxidants have been widely used as a general approach to ameliorate excessive oxidative stress both in animal models and humans. For example, resveratrol has been shown to extend the lifespan of various species, and to be effective at improving the health and survival of mice on a high-calorie diet (Baur et al. 2006).
To date, the development of effective treatments for oxidative stress and certain diseases associated with oxidative stress using natural or synthetic anti-oxidants has been problematic. Stringent scientific proof for the efficacy of natural anti-oxidants has not been established (Droge et al. 2001). Some of the notable limitations for using natural anti-oxidants as therapeutics include low potency and fast turnover during metabolism. In contrast, the development of synthetic anti-oxidants has been inhibited by safety concerns. Nevertheless, some progress has been made in this direction. Modification of a natural anti-oxidant has been performed to enhance its potency (Keum et al. 2007). In addition, synthetic mimics of superoxide dismutase (SOD) and catalase have been shown to be effective in rodent models of ischemia and Parkinson's disease (Peng et al. 2005). Even more encouragingly, a class of nitron-free radical trap agents, alpha-phenyl-N-tert-butyl-nitron (PBN) and disodium 2,4-disulfophenyl-N-tert-butylnitrone (NXY-059), have been shown to be Potent neuroprotective agent (Maples et al. 2004), and have demonstrated anti-cancer activity in hepatocellular carcinoma through its anti-inflammatory property (Floyd 2006).